A video codec is a well known device which includes a coder that converts analog video signals to compressed digital data for transmission to a remote location. The codec also includes a decoder that expands compressed digital data received from a remote location and converts the expanded digital data to analog video signals for display. Video codecs are used in a number of duplex application such as video teleconferencing (VTC) systems. Typically, the coder accomplishes the digital compression by implementing various coding algorithms. These algorithms remove redundant information in the video image in order to reduce signal bandwidth sufficiently to allow transmission to the remote location over a relatively low cost transport such as standard digital telephone circuits. A reversal of the coding process is employed by the decoder to reconstruct the original video image.
A typical television signal, when digitized, produces a data stream of about 100 to 150 megabits per second (Mbps) before compression. Compressed video data streams usually range from 56 kilobits per second (kbps) to 45 Mbps. The receiving codec expands or decompresses the digital signal, then converts it back to an analog video format for display.
The process of compressing and decompressing the signal introduces defects, called motion artifacts in the received video image. These artifacts are readily observable when the video signals are displayed. Such well known motion artifacts include (1) blocking, where the received video image has rectangular or checkerboard patterns not present in the original image; (2) blurring or smearing, where the received video image has lost edge detail present in the original; (3) image persistence, where an old or previous image remains on the monitor; (4) jerkiness, where the original smooth continuous motion is perceived as a series of snapshots.
For a given codec, higher compression ratios (lower transmission rates) will generally produce more noticeable motion artifacts. For example, when a signal is compressed to 56 kbps (a compression ratio of over 1000:1), the resulting motion artifacts are obvious to observers. At the lower compression ratio (about 3:1) that produces 45 Mbps transmission, most observers cannot detect the motion artifacts.
Video test equipment such as test signal generators, vector scopes, and waveform monitors are useful for evaluating the quality of analog video signals. However, they don't provide a good measure of motion artifacts produced by codecs. At the present time, no test equipment is available to provide an objective measure of a codec's performance; so subjective panel evaluations have been used.
In order to insure a certain quality of service it is desirable to have a method of objectively measuring the performance of codec since it is the critical component of digital video teleconferencing. Prior attempts to obtain a measure of the performance of the codec have not been entirely successful due to the complexity and attendant expense involved. For example, it has been proposed to use various motion graphics or recorded video scenes as the image to be compressed, and to compare individual pixels of the video image before and after being processed by the codec. See "Subjective and objective testing of video teleconferencing/videophone systems" by Schaphorst and Bodson; and "The development of objective video quality measures that emulate human perception" by Voran published in Proceedings of IEEE GLOBECOM '91 at pages 1771-1775 and 1776-1781 respectively.